Reference voltages are needed in equipment such as power supplies, current supplies, panel meters, calibration standards, data conversion systems, and the like. Bandgap reference circuits are typically chosen to produce reference voltages due to their ability to maintain stable output voltages that vary little with temperature and supply voltage.
A typical bandgap reference circuit 10 is shown in FIG. 1. Circuit 10 includes an amplifier 11 and a bandgap voltage generator 12. The output of the bandgap reference circuit (at node Vbgr) stabilizes according to the following equation:
                                                        Vbgr              =                                                Vbe                  ⁢                                                                          ⁢                  2                                +                                                      (                                                                  Vbe                        ⁢                                                                                                  ⁢                        1                                            -                                              Vbe                        ⁢                                                                                                  ⁢                        2                                                              )                                    *                                      (                                          1                      +                                              2                        *                        R                        ⁢                                                                                                  ⁢                                                  1                          /                          R                                                ⁢                                                                                                  ⁢                        2                                                              )                                                                                                                          =                                                Vbe                  ⁢                                                                          ⁢                  2                                +                                                      (                                          Vt                      *                                              ln                        ⁡                                                  (                          n                          )                                                                                      )                                    *                                      (                                          1                      +                                              2                        *                        R                        ⁢                                                                                                  ⁢                                                  1                          /                          R                                                ⁢                                                                                                  ⁢                        2                                                              )                                                                                                          (        1        )            where Vbe1 and Vbe2 are the base to emitter voltages of bipolar junction transistors (BJTs) 15 and 16, respectively, and R1 and R2 are the resistances of the resistors 13 and 14 respectively. Vt is the thermal voltage, which is approximately 25.853 milliVolts (mV) at a temperature of 300 degrees Kelvin (˜26.84 degrees Celsius), and n is the ratio of the current density of BJTs 15 and 16.
In equation (1), the first term on the right hand side has a negative temperature coefficient, while the second term on the right had side has a positive temperature coefficient. An almost zero temperature coefficient can be obtained by setting a proper ratio between the first and the second terms on the right had side of the equation.
An intrinsic problem with a bandgap reference circuit such as circuit 10 is that it has two stable states. A first stable state is the normal operational state, where Vbgr is equal to about 1.25 Volts (V). The second stable state is the zero-current state, where Vbgr is equal to 0 and Vbias is equal to 0.
To prevent the reference circuit 10 from staying in the zero-current state, a startup circuit, such as startup circuit 23 shown in FIG. 2, is normally added to the bandgap reference circuit. The startup circuit may include a resistor and several diode-connected n-channel metal oxide semiconductor field effect transistors (NMOSFETs). In circuit 23, the voltage at terminal 24 is higher than Vt1+Vt2, where Vt1 and Vt2 are the threshold voltages of transistors 18 and 19, respectively. This ensures that Vbias, Vbgr, and the voltage at node 25 will be pulled to at least Vt1+Vt2−Vt3, where Vt3 is the threshold voltage of the transistors 20, 21, and 22. Therefore, using the startup circuit 23, the bandgap circuit will be powered up to the normal operational state.
The startup circuit 23 has two major drawbacks. First, if the power supply voltage Vcc is less than Vt1+Vt2, then Vbias, Vbgr, and the voltage at node 25 can only be pulled up to a level of Vcc−Vt3. For example, if Vcc=1.6 V, and Vt3=1.0 V, Vbias, Vbgr, and the node 25 voltage can be pulled to 0.6 V, which is not enough to turn on the NMOSFETs 26, 27, 28, and 29, and BJTs 15 and 16 provided the threshold voltages of those devices are larger than 0.6 V, since typical threshold voltages for such devices are approximately 0.7 V. Therefore, the bandgap reference circuit 10 will stay in the zero-current state. Second, the startup circuit 23 consumes power during the normal operation of the circuit 10. This is unacceptable, especially if the circuit 10 is used for portable devices, which have stringent power consumption requirements of a few microwatts.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a startup circuit for low power circuits.